Box hole drill steel

ABSTRACT

Box hole drill steel comprises pipe of short length, i.e. of the order of one yard, to be easily handled with its axis transverse to a mine drift, and of large circumference, i.e. larger than its length, to provide stiffness for straight boring and to transmit large torque for boring a big hole, e.g. 5 feet in diameter, the pipe including a plain carbon steel body with alloy steel pin and box threaded tool joints welded to its ends, each thread being straight (untapered) to minimize hoop tension and being of modified buttress configuration for strength, the thread having a steep pitch and a double lead, being of less than 360* circumference but greater than 180* to insure a tight and stable connection, the pin being provided with a shoulder to engage the box mouth to transmit torque and axial compressive force for turning and loading the bit, the body of the pipe having a handling tube at each end extending diametrically therethrough and welded to the body at each end, the tube providing means by which the pipe can be engaged for lifting, lowering, turning and holding.

' United States Patent 1191 Rodgers Nov. 4, 1975 BOX HOLE DR ILL STEEL Primary Examiner-Wemer H. Schroeder Inventor: Otis Ned Rodgers, Midland, Tex. Assistant Examiner-M08 Chen Attorney, Agent, or Firm-Murray Robinson [73] Assignee: Smith International, Inc., Midland,

Tex. [22] Filed: June 14, 1974 [57] ABS CT Box hole drill steel comprises pipe of short length, i.e. [21] Appl' 479307 of the order of one yard, to be easily handled with its axis transverse to a mine drift, and of large circumfer- [52] U.S. Cl. 285/; 85/46; /85; ence, i.e. larger than its length, to provide stifiness for 403/19; 403/343; 285/39; 285/334; 285/390; straight boring and to transmit large torque for boring 285/401 a big hole, e.g. 5 feet in diameter, the pipe including a [51] Int. Cl. F16L 55/00 plain carbon steel body with alloy steel pin and box [58] Field of Search 285/390, 39, 333, 334, threaded tool joints welded to its ends, each thread 285/376, 401, 38, 114, 115, 355; 166/775; being straight (untapered) to minimize hoop tension 175/52, 53, 85; 403/ 16, 19, 296, 306, 343, and being of modified buttress configuration for 361; 85/46 strength, the thread having a steep pitch and a double lead, being of less than 360 circumference but greater [56] References Cited than 180 to insure a tight and stable connection, the UNITED STATES PATENTS pin being provided with a shoulder to engage the box mouth to transmit torque and axial compressive force 3:238:53 211323 filin iiiiiii1113133111:3111113511178; for .hhhihg hhdihg hhe hh the body P Pipe 3,355,192 11/1967 Kloesel, Jr. et a1. 285/334 x havmg a handllng tube at each end extendlng dlamet- 3,399,738 9/1968 Haspert 175/53 rically ugh and elded to the body at each 3,442,536 5/1969 Fowler 285/376 X end, the tube providing means by which the pipe can be engaged for lifting, lowering, turning and holding.

7 Claims, 7 Drawing Figures a aw am qr 7T7 BOX HOLE DRILL STEEL CROSS REFERENCE TO RELATED PATENTS AND PUBLICATIONS A related application also entitled Bos Hole Drill Steel is being filed concurrently herewith, assigned to the same assignee as the present application.

The disclosures of the following United States patents and the aforementioned concurrent application are incorporated herein by reference:

3,355,192 Kloesel, et al.

3,754,609 Garrett 3,067,563 McCool 2,636,753 Griffin 3,399,738 Haspert Subterranean Model UR-60 Box Hole Machine, a brochure published by Subterranean Tools, Inc.

BACKGROUND OF THE INVENTION This invention relates to drill steel and more particularly to box hole drill steel and connections therefor.

Heretofore it has been known to use flanged couplings for connecting lengths of box hole drill steel. This makes connection and disconnection of the lengths of drill steel a tedious time consuming process.

Possible difficulties with conventional threaded pipe connections if used for box hole drill pipe would be excessive hoop tension due to thread taper when the connection is made up tight enough to prevent accidental loosening, difficulty in applying wrenches for screwing the connections together due to limited space, undue length of connection compared to pipe length, and time required for making several revolutions of the pipe during make up and break out.

Some of the foregoing difficulties have been met in connections for other types of pipe. For example relative to fast make up and break out US. Pat. No. 3,442,536 to Fowler describes wellhead apparatus including a breech block type connection employing no lead teeth. The references cited in the Fowler patent disclose fast make up interrupted thread type connections. A threaded rotary shouldered connection having rugged modified buttress threads with double lead and a steep pitch for fast make up and break out suitable for use in mine raise drilling is disclosed in US. Pat. No. 3,355,192 to Kloesel, et al. The theory of rotary shouldered connections for oil well drill pipe is explained at some length in US. Pat. No. 3,754,609 to Garrett. The use of straight threads in conjunction with a torque transmitting shoulder is disclosed in US. Pat. Nos. 2,636,753 and 2,825,585 to Griffin, the connection being between a length of drill pipe or a drill collar and a tool joint replaceably shrink fitted thereto. Tubular drill rod for blast hole drilling having threaded ends and provided adjacent each end with a transverse sleeved hole for reception of a holding bar is shown in US. Pat. No. 2,849,212 to Robbins. Guide pipe for a box hole drill, the pipe having transverse holes but without sleeves therein is shown in US. Pat. No. 3,399,738 to Haspert.

Some idea of the practical differences between the foregoing prior art constructions and the present invention will appear when it is noted that ordinary oil field drill pipe is usually of the order of 30 feet long and a foot or two in circumference for downwardly drilling 5000 foot deep holes of less than a foot in diameter, blast hole drill rod is similar to oil field drill pipe but usually somewhat shorter, e. g. twenty feet in length and used to drill downwardly a hole of about a foot in diameter and of a hundred feet or less in depth, raise drill steel is used to drill, usually upwardly from above through a pilot hole, a raise or tunnel of perhaps feet in length and of the order of three to fifteen feet in diamter, the steel itself having a circumference of a couple of feet and a length of ten feet or so, whereas box hole drill steel to which the present invention pertains is used in drilling upwardly from below and without a pilot hole an inclined passage of about five feet in diameter and perhaps thirty feet long, the length of the steel being of the order of one yard and having a large circumference, i.e. larger than its length. The foregoing is summarized in the following table of typical dimensions:

Different utilities create different combinations of requirements to be met and problems to be overcome; the present construction meets the requirements of box hole drilling and advantageously overcomes the problems of prior box hole steel constructions discussed above. Other objects and advantages of the invention will appear from the following description.

SUMMARY OF THE INVENTION According to the invention box hole steel comprises pipe of short length, i.e. of the order of one yard or so, and having a circumference to length ratio greater than one, the ends each being provided with a rotary shouldered tool joint having straight, modified buttress, double lead, high pitch threads of less than 360 but greater than 180 circumferential extent, i.e. the circumferential extent of each thread is of the same order as the length of the pipe body to which the tool joints are connected and greater than the arc subtended by, e.g., the length of the adjacent bar receiving tube, the extent preferably being 270. Holding means are provided for supporting a string of the pipe in the hole against axial movement while pipe lengths are being added or removed and to prevent rotation of the pipe relative to the hole or the drilling head when making and breaking connections. The present application is one of two related applications in one of which the holding means includes a bar receiving tube transecting the pipe near one or preferably both ends and such holding means forms part of the invention, such holding means not being claimed in the other (the present) application.

BRIEF DESCRIPTION OF DRAWINGS For a detailed description of a preferred embodiment of the invention, reference will now be made to the accompanying drawings wherein:

FIG. 1 is a schematic sectional view of a gold mine with a box hole drill operating therein employing box 3 hole drill steel in accordance with the invention;

FIG. 2 is a sectional view of a length of box hole drill steel embodying the invention;

FIG. 3 is a schematic isometric view of the tool joint threads in accordance with the invention;

FIG. 4 is a sectional view showing the details of the pin tool joint of the drill steel in accordance with the invention;

FIG. 5 is a sectional view showing the thread form of the pin tool joint of the drill steel in accordance with the invention;

FIG. 6 is a sectional view showing the details of the box tool joint of the drill steel in accordance with the invention; and v FIG. 7 is a sectional view showing the thread form of the box tool joint of the drill steel in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1 there is shown a mine drift 11 in which is disposed a drill 13. A drill string 15 extends into box hole 17 which, as indicated by the broken lines, will ultimately extend up to a drift, sublead, stope or box 19 in which a man will mine gold. The drill string includes a plurality of lengths of drill steel 23. Certain pieces of drill steel may have on their exteriors various structures such as the wall engaging means of stabilizer 25, the detritus diverter or shield 27, and the multiple cutters of bit 29. The term drill steel as used herein is intended to cover all drill string elements employing drill steel in accordance with the invention regardless of whether or not combined with such other structure. It is also to be noted that the last element of the drill string, namely the drill bit, embodies the invention at only one end, the other end being adapted for contacting the end of the hole rather than for connection to other drill steel, but to a certain extent the bit body may incorporate the present invention.

5 each end are holding or rod receiving tubes 35. Typi-' 40 The above described apparatus is operated to drill a box hole by rotating the drill string with the drill 13 and advancing the bit in the hole to maintain pressure against the end of the hole. The detritus falling away from the bit drops down the inclined hole 17 and into trough and then into the drift for ultimate disposal.

Referring now to FIG. 2, there is shown one of the lengths of drill steel 23 included in the drill string 15. Steel 23 includes a tubular body 31 of plain carbon steel, having pin and box alloy steel tool joints 33, 35 welded to its ends. For a discussion of the merits of alloy steel tool joints see U.S. Pat. No. 3,067,563 to M. G. McCool.

The body 31 typically has a thickness of 1/2 inch. Other typical dimensions of the drill steel would be as shown in the following table:

Overall External Ratio of Outside Length Circumference Circumference Diameter in Inches in Inches to Length 13 24(to 36) 4I 1.7 (to l.15) 14 /4 36 46 1.3 20 48 63 1.3

drilling a straight hole and enough strength to transmit the large torque required for drilling a large diameter cally the tubes are made of steel and have an inner diameter of about three inches, large enough to receive holding rods, and a wall thickness of 3/4 inches, strong enough to transmit the required forces and torques between the drill steel and holding rod. In connection with some drilling machines only one holding tube may be necessary for making, breaking, and supporting the drill steel in the drill string, but two are preferred in any case because these tubes provide transverse structural support for the large circumference pipes. Each holding tube 35 extends through radial holes 37 in the body 31, being welded at each end to body 31 by annular welds 39 extending around the tube ends between the tube and the outside of the body. If desired additional annular welds between the tube and inside of the body may be used. The tubes are adapted to receive holding rods extending therethrough and through the frameof drilling machine or drill 13 (FIG. 1), e.g. to support the drill string axially and against rotation when the lowermost piece of drill steel, adjacent the drill, is being inserted and connected or disconnected and removed. One or more holding rods can also be used to lock the lowermost drill steel to the drill head or to anchor it to the drill frame in the course of such operations, as will be described in more detail hereinafter. The drill steel is thus adapted for use in a drilling machine of the type disclosed in the above referenced brochure of Subterranean, modified. to use threaded connections, as shown in FIG. 1, although the steel could be used with other similar drills or drilling machines. The steel is not intended to carry drilling fluid but it is understood that users, not the present inventors, have added a small internal pipe to convey water to the bit for dust suppresslon.

Turning now to the tool joints 33, 35, each includes a short weld neck 51 of the same inner and outer diameter as body 31 adapted for welding to the body 31 by annular welds 53. Pin tool joints 33 further includes an exteriorly threaded pin 55 of smaller outer diameter than body 31 and a transition zone 57. The outer end of the transition zone provides an annular shoulder 59 for engagement with the end shoulder of another length of drill steel, similar toend shoulder 61 on the box tool joint 35. The box tool joint 35 further includes an interiorly threaded box 63 adapted to engage withthe pin of another tool joint like pin 55 of pin tool joint 33. The outer diameter of box 63 is the same as that of body 31 and the wall thickness is greater than that of the body 31, being about /8 inch exclusive of the thread. The pin 55 is only slightly thicker, being about inch exclusive of the thread (see FIG. 4).

The pin and box tool joints are untapered and are provided with threaded portions 65, 67. The threads are helical and of modified buttress configuration with 45 degree unengaged flank angles and 15 degree pressure flanks. The threads are double lead, that is, there are two pin threads 65A, 65B and two box threads 67A, 678, the turns of one thread at each end lying in between the turns of the other thread at that end. The thread lead is one inch (two threads per inch double lead) and since the threaded portions of the pin and box extend axially only one inch, there is room for but one turn of each thread. The ends of each turn would I be imperfect and such portions are removed leaving only about turn or a circumferential extent of about 270. This leaves a gap of about 90 or A turn. Since the threads are double, the corresponding ends of the threads are displaced 180. This results in the 90 gap in one thread having the same azimuthal position as the midport of the adjacent thread. This provides stability and prevents one length of pipe from canting relative to the adjacent length of pipe even though the axial length of the threaded portions (1 inch) is less than of the outer diameter of the pipe. The relative positions of the threads at each end of the pipe are exemplified in FIG. 3.

Dimensional details and tolerance for the pin and box tool joints and threads preferred for a box hole drill steel of 14% inch diameter are shown in FIGS. 4-7. Comparing FIGS. 4 and 6 it will be noted that the pin thread crest diameter is less than the box thread valley diameter so that the pin thread crests do not engage the box thread valleys. Likewise, the diameter of the pin thread valley is less than the diameter of the box thread crests so that the box thread crests do not engage the pin thread valleys. This clearance is characteristic of a straight thread since without a taper there is no way to expand the box over the pin as would be required if there were to be contact. It will also be seen that the thread on both pin and box are considerably narrower than the valleys in which they are received so that dirt will not interfere with proper make up.

The range of dimensions is preferably the same as in Unengaged Flank Angle: Lead: Taper:

30 to 60 degrees V4 to 1 /4 Zero i W per foot The threads are quite coarse for ruggedness, but if a somewhat finer thread were used, higher thread multiplicity than two could be used without the lead becoming excessive.

Referring now particularly to FIG. 2, the pin tool joint has an annular unthreaded portion 71 between the shoulder 59 and the threaded portions 65. Also the box tool joint has an unthreaded portion 73 between the end shoulder 61 and threaded portion 67. These unthreaded portions are respectively stretched and compressed when the tool joints are made up tighter after the shoulder 59 of a pin joint engages the mouth 61 of a box joint. The resultant frictional engagement of the shoulders 59 and 61 is sufficient to transmit all or most of the torque between the pin and box tool joints without the threads being subjected to torque. The connection is therefore a rotary shouldered connection.

To effect torque transmission through the shoulder of a rotary shouldered connection, as is intended, the connection must be made up tight enough to create the necessary axial compression of the unthreaded portion or pin neck 71 and the unthreaded portion or box mouth 73. For example in the illustrated embodiment a .make up torque of 40,000 foot pounds is suitable. To

insure that the drill string can be broken apart as is necessary to remove it from the box hole, break out torque should be less than makeup torque. The connection of the present invention meets this requirement with a breakout torque at 36,000 foot pounds when made up with 40,000 foot pounds. Break out torque is thus only of make up torque.

Reference will now be made once more to FIG. 1 to describe the operations performed when pipe lengths are to be added to the drill string for continuing forward drilling and when pipe lengths are to be removed from the drill string when the bit is to be retracted from the hole, e.g. when drilling has been completed or when the drill bit or a cutter thereof is to be replaced. It is to be understood that in the preferred method of use the drill pipe is run in the hole with the box up and pin down.

If it is desired to add a joint of pipe, the machine being at the upper limit of its travel, the lower holding tube of the lowermost joint in the string is pinned to the drilling machines frame with a bar 81. The machine is operated in reverse to completely unscrew the machine head box 82 from the pin of the lowermost joint. The machine head is then retracted to its lowermost position. Handling arm 83 loaded with another pipe joint swings the additional joint into position between the head box and the pin at the lower end of the drill string. The head is raised and rotated to screw the head box to the pin of the added pipe joint and to screw the box of the added pipe joint to the pin at the lower end of the drill string. The rod 81 is then removed and drilling can proceed.

When it is desired to remove the drill string from the hole, this is accomplished one joint at a time as follows. With the head at its upper limit of travel the lower holding tube of the lowermost joint in the string is pinned to the machine frame with bar 81. The machine is then operated in reverse to partially unscrew the head box from the drill string pin. The bar 81 is then removed and the head retracted to its lowermost position, the drill string lowering also. The head is then pinned to the lower holding tube of the lowermost joint of pipe and the upper holding tube of the next to lowest joint of pipe is pinned to the machine frame with bar 81. The machine is operated in reverse to unscrew the lowermost joints box from the pin of the next to lowermost joint. The handling am 83 is then moved into position around and gripping the lowermost joint of pipe. The machine is then operated in reverse to unscrew the low errnost joint of. pipe which is held against rotation by the handling arm. The handling arm and the lowermost joint are then moved away and the head elevated and loosely screwed onto the lowermost pin of the remaining portion of the drill string. The bar 81 is removed and the drill string lowered to the lowermost position of the machine. The procedure is repeated until all of the desired portions of the drill string have been removed.

A feature of the invention is the fact that because of the thread configuration, less than one revolution of the drilling machine is required for make up and break out of pipe joints.

As previously mentioned one of the problems encountered in box hole drilling is the limited space available. The drilling apparatus, including both the drilling machine or drill and the drill pipe must have a maximum extent sufficiently short to enable it to go down the mine shaft in the elevator cage, then along the low ceilinged horizontal drift, go through pressure doors, around bends, and along rails. Blasting disturbs the walls of the mine shaft and drifts are made as small as practicable. It is for these reasons that the box hole pipe is short, e.g. of the order of one yard plus or minus 50%. During drilling the drill string is at an angle and 7 tends to bend over. To keep the string as straight as possible it must be stiff. Stiffness varies with the fourth power of the circumference, so a large circumference pipe is desirable. It is the combined result of these two requirements that result in the pipe having a circumference to length ratio greater than unity as previously set forth. 7 7

During drilling the drill string is in compression. Axial compression is transmitted by the shoulders of the rotary shouldered'connection, rather than through the threads, and, as previously explained, torque is also transmitted through the shoulders. Therefor the threads do not take the drilling loads. This facilitates use of the 270 threads. It is also to be noted that the use of a double lead provides thread support on the entire 360 even though each thread is only 270 in extent. Finally, as explained in the foregoing Griffin patent, only the end turns of a thread take much of the axial load. For these reasonsthere is adequate thread strength provided by the fast make up thread of the present invention.

Although the invention has been shown and described with respect to drill steel operating with the box up and pin down, and a box on the drill head and a pin on the drill bit, the steel could be run box down, the head could be provided with a pin, and the bit could be provided with a box and by use of suitable subs, any combination of these pin and box arrangements can be used, e. g. drill head pin or box, pipe box up or down, bit with pin or box.

While a preferred embodiment of the invention has been shown and described, modification thereof can be made by one skilled in the art without departing from the spirit of the invention.

I claim: 1. Threaded connection adapted for repeated make up and break out during use comprising a pin tool joint of generally tubular configuration including an annular shoulder and a pin extending away from the shoulder, the pin including an unthreaded portion adjacent the shoulder and a threaded portion adjacent said unthreaded portion and axially away from said shoulder, and

a box tool joint of generally tubular configuration including an annular shoulder and a box extending away from said shoulder, the box including an unthreaded portion adjacent the last said shoulder and a threaded portion adjacent the last said unthreaded portion and axially inward from said shoulder, I

the box threaded portion being adapted to engage the pin threaded portion and said shoulders being adapted to engage when said threaded portions are made up, said tool joints being made of elastic metal such as steel whereby final makeup of the threaded portions axially stresses said unthreaded portions, one in tension and the other in compressron,

said threaded portions each having double lead helical threads each of greater than 180 but less than 360 extent leaving a circumferential gap in each thread, the gaps being disposed 180 apart, the gap in one thread on each portion having the same azimuthal position as the mid part of the otherthread of that same threaded portion, the taper of said threads being zero plus or minus /2 inch per foot, whereby said connection is adapted for fast make-up and break-out yet is stable in resisting relative cant ing of the pin and box tool joints when made up. 2. Threaded connection according to claim 1 wherein the circumferential extent of each thread is 270 plus or minus ten degrees and the ends of each thread are substantially perfect.

3. Threaded connection according to claim 1 wherein the ratio of the length of each threaded portion to the outside diameter of the tool joint is less than ten percent.

4. Box hole drilling apparatus element adapted for connection to another element of such apparatus, the

first said element including a steel tool joint adapted to.

form part of a rotary shouldered connection, said joint including in axial sequence a shoulder, an unthreaded portion, and a threaded portion,

said threaded portion having double lead helical threads each of a circumferential extent of 270 plus or minus 10 with the circumferential gaps in the threads being disposed diametrically opposite 1 each other, the ends of the threads being substantially perfect and the entire threads including at least substantially full crest width, the taper of the threads being zero plus or minus inch per foot,

the length of the threaded portion being less than ten percent of the outside diameter.

5. In a rotary shouldered connection comprising pin and box tool joints having double lead helical threads of modified buttress configuration with a pressure flank angle between 10 and 20 and, an unengaged flank angle between 30 and 60 and a lead of between inch and 1% inches, the break out torque being less than makeup torque, the improvement for use with box hole steel having a circumference to length ratio. greater than unity comprising the thread taper being zero plus or minus /2 inch per foot, the circumferential extent of each thread being less than 360, the circumferential gaps in the two threads being diametrically op less than 10 percent of the outside diameter of the tool joint, the. thread lead being 1 inch plus or minus 10 percent.

7. Combination according to claim 6 wherein the threads each have a circumferential extent of 270 plus.

or minus 10 percent, the taper is substantially zero, and the pressure and disengaged flank angles are respec tively 15 and 45 plus or minus l0 percent.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT N0. 3,917,321

DATED November 4, 1975 INVENTOR(S) I Otis Ned Rodgers It is certified that enar appeaTs in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 6, change "B to Box.

Column 2, line 7, change "diamter" to diameter.

Column 5 in the table of dimensions, change "3/4' to 1-1/4 to 3/4" to l-l/4"-.

Column 7, line 12, change "therefor" to Signed and Scaled this Tenth Day of August 1976 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Commissioner of Parents and Trademarks Arresting Officer 

1. Threaded connection adapted for repeated make up and break out during use comprising a pin tool joint of generally tubular configuration including an annular shoulder and a pin extending away from the shoulder, the pin including an unthreaded portion adjacent the shoulder and a threaded portion adjacent said unthreaded portion and axially away from said shoulder, and a box tool joint of generally tubular configuration including an annular shoulder and a box extending away from said shoulder, the box including an unthreaded portion adjacent the last said shoulder and a threaded portion adjacent the last said unthreaded portion and axially inward from said shoulder, the box threaded portion being adapted to engage the pin threaded portion and said shoulders being adapted to engage when said threaded portions are made up, said tool joints being made of elastic metal such as steel whereby final makeup of the threaded portions axially stresses said unthreaded portions, one in tension and the other in compression, said threaded portions each having double lead helical threads each of greater than 180* but less than 360* extent leaving a circumferential gap in each thread, the gaps being disposed 180* apart, the gap in one thread on each portion having the same azimuthal position as the mid part of the other thread of that same threaded portion, the taper of said threads being zero plus or minus 1/2 inch per foot, whereby said connection is adapted for fast make-up and breakout yet is stable in resisting relative canting of the pin and box tool joints when made up.
 2. Threaded connection according to claim 1 wherein the circumferential extent of each thread is 270* plus or minus ten degrees and the ends of each thread are substantially perfect.
 3. Threaded connection according to claim 1 wherein the ratio of the length of each threaded portion to the outside diameter of the tool joint is less than ten percent.
 4. Box hole drilling apparatus element adapted for connection to another element of such apparatus, the first said element including a steel tool joint adapted to form part of a rotary shouldered connection, said joint including in axial sequence a shoulder, an unthreaded portion, and a threaded portion, said threaded portion having double lead helical threads each of a circumferential extent of 270* plus or minus 10* with the circumferential gaps in the threads being disposed diametrically opposite each other, the ends of the threads being substantially perfect and the entire threads including at least substantially full crest width, the taper of the threads being zero plus or minus 1/2 inch per foot, the length of the threaded portion being less than ten percent of the outside diameter.
 5. In a rotary shouldered connection comprising pin and box tool joints having double lead helical threads of modified buttress configuration with a pressure flank angle between 10* and 20* and an unengaged flank angle between 30* and 60* and a lead of between 3/4 inch and 1 1/4 inches, the break out torque being less than makeup torque, the improvement for use with box hole steel having a circumference to length ratio greater than unity comprising the thread taper being zero plus or minus 1/2 inch per foot, the circumferential extent of each thread being less than 360*, the circumferential gaps in the two threads being diametrically opposite each other, and the ends of each thread being substantially perfect and the entire threads including at least full crest width.
 6. Combination according to claim 5 wherein the axial extent of the threaded portion of each tool joint is less than 10 percent of the outside diameter of the Tool joint, the thread lead being 1 inch plus or minus 10 percent.
 7. Combination according to claim 6 wherein the threads each have a circumferential extent of 270* plus or minus 10 percent, the taper is substantially zero, and the pressure and disengaged flank angles are respectively 15* and 45* plus or minus 10 percent. 